Towards Terahertz technologies

In short: Using a novel Optical Hall technique, we have been able to determine, for the first time, how fast the free electrons can travel in thin aluminum-rich AlGaN layers and how they scatter. We also studied stacks of graphene layers and could identify extremely mobile electrons in perfect and non-interacting layers. In addition, channels with slow and very slow electrons were identified, where the reduced mobility is caused by interactions among graphene layers as well as with the underlying substrate. These results are prerequisites for THz device design and operation.

Novel processor-technologies that are operating at Terahertz frequencies (THz), a thousand times faster than current silicon devices will revolutionize telemedicine, life science, ultra-fast, high-resolution telecommunication, and intelligent automotive transportation, for example. However, conventional semiconductors - such as silicon - have reached their limits and new materials need to be developed in order to achieve THz frequencies of electronic devices.

We focus our efforts on developing two-dimensional materials, such as graphene and the two-dimensional electron gas generated in a very thin channel at the interface of two different nitride layers (GaN/AlGaN or InAlN/GaN for example). These materials possess the desired transport properties surpassing any other known material and if used as active layers in electronic devices THz frequencies can be in principle reached. What remains is to determine and optimize the transport properties of graphene and thin nitride films.

In 2013 we have explored new frontiers in materials research by using a unique optical technique that we term the Optical Hall effect. This technique is based on detecting changes in the polarization of light when it is reflected or transmitted by the material. The Optical Hall effect is revealed by spectroscopy of such polarization changes, i.e. spectroscopic ellipsometry, in combination with a magnetic field. The unprecedented capabilities of the Optical Hall effect are the gathering meaningful information about electronic and spin transport in individual layers of a multilayered structure with no physical contacts that can result in surface damage or sample contamination.

In our work we have been able to determine for the first time how fast the free electrons can travel in thin aluminum-rich AlGaN layers and how they scatter, which is a prerequisite for THz AlGaN device design and operation. We also studied samples containing stacks of tens of individual graphene layers - single carbon atoms sheets arranged in a honeycomb lattice - in which the electrons travel only 300 times slower than the speed of light in vacuum. Using the Optical Hall effect technique we have been able to identify three distinct groups of free electrons in the graphene stacks. First the properties of the extremely mobile electrons, which are suitable for transistors and electronic processors capable of operating at

Terahertz frequencies, have been determined and associated with perfect single graphene sheets that do not interact between each other. However, an additional contribution from “slow” electrons has been also identified and related to graphene sheets in the stack that interact with each other and form defects similar in structure to graphite. Finally, a third group of “very slow” electrons has been found and associated with the interface layer with the silicon carbide substrate on top of which the graphene stack is grown. Current efforts include use of the Optical Hall effect measurements to minimize the graphitic defects and grow graphene layers with extremely mobile electrons over a large area of two-inch diameter substrates.

Details of the research are described in the following two journal articles: